If you click on the creature it has a rarity rate (Not sure if it is accurate) but if so I think it is the lower the number. Like the Akk Dog Rarity: 0, and I have found many of them but Krayt Dragon Rarity: 5 I have seen once.

I guess you would also have to take into consideration of the terrain since they also have a % rate of creatures spawned, then look at which creatures spawn on what terrain and then the rarity rate of said creature compared to the creature spawn % of said terrain.

So, Spice Grabs are as rare as Space Slugs? Anyway, like i said, saw too many Space Slugs to my liking. It is not problem when creature aggression is off, but later...
Maybe it was luck, but maybe those numbers need a little tweak.